When the Sun goes down, the air gets cooler. As Earth's rotation carries your part of the world into darkness, the heat that built up during the daytime radiates into space. The exact temperature drop varies, but a difference of 20 to 30 degrees is fairly typical.

On worlds without air, the change is much more drastic. On the Moon, for example, the temperature can swing by close to 500 degrees Fahrenheit. And on Mercury, the closest planet to the Sun, it can be close to a thousand degrees.

But on Venus, the next planet outward, there's no difference at all. Daytime and nighttime temperatures are identical. And as you might have guessed, it's because Venus has a thick atmosphere.

Venus's "air" consists almost entirely of carbon dioxide, and it's about 90 times denser than Earth's air. What's more, it's topped by unbroken layers of clouds made of sulfuric acid.

This thick atmosphere acts as a blanket -- it doesn't let the heat escape into space. So after the Sun sets, there's no way for the planet to cool off. The heat continues, day and night, around the entire planet. And when we say "heat," we aren't kidding. The temperatures on Venus are close to 900 degrees Fahrenheit -- hot enough to melt lead.

Although it's a hellish place to visit, Venus is a great world to admire from afar. It's the brilliant "evening star," low in the west shortly after sunset. Its cooler neighor, Mercury, stands a little to its right.